Sharks are widely thought to use the earth’s magnetic field for navigation but most evidence for this putative compass sense is circumstantial. For example, scalloped hammerhead sharks (Sphyrna lewini) orient to seamounts where geomagnetic anomalies exist, and both tiger sharks (Galeocerdo cuvier) and blue sharks (Prionace glauca) can swim in straight lines for long periods across open ocean. Captive studies have demonstrated that stingrays orient to electric fields similar to those produced by ocean currents moving through the earth’s magnetic field but to date, there is no empirical evidence that sharks can detect magnetic fields per se. We used behavioral conditioning to demonstrate that sharks can detect changes in the geomagnetic field.

Research Questions

We are addressing the following specific questions:

Can sharks detect changes in the Earths magnetic field?

What sensory system do sharks use to detect magnetic fields?

What components of the earths magnetic field are detected by sharks?

Methods

Six sandbar sharks (Carcharhinus plumbeus) and one scalloped hammerhead shark were housed in a 7 m diameter tank surrounded by an 8 m diameter coil consisting of 100 turns of 18 AWG copper wire. Supplying power (13.8 VDC, 1.5 A) to the coil produced a vertical magnetic field inside the tank with a total field strength ranging from 100 µT at the perimeter to 25 µT at the centre. Ambient magnetic field intensity was 36 µT. Therefore, the altered field was between 0.7 and 2.8 times the total intensity of earth's magnetic field in Hawaii.

Sharks were conditioned by pairing activation of the artificial magnetic field with presentation of food at a 1.5 x 1.5 m square target marked on the tank floor. Following conditioning, the behavioral bioassay compared the number of times sharks entered the target (feeding) area under normal magnetic field conditions with the number of passes through the target when the artificial field was activated. Responses were recorded by a video camera suspended above the target area. Each trial consisted of a 21 minute observation period; 10 minutes before turning the field on, one minute with the artificial field on and 10 minutes after turning the field off. Eleven trials were conducted over a six-week period. No food was presented during experimental trials but conditioning was reinforced between experiments. To preclude erroneous conditioning to extraneous cues, the electronic switching equipment was physically isolated from the test tank and scientific personnel were not visible to the sharks in the tank.

Watch Video: Click the image above for a Quicktime video (10 MB) of these experiments

Results

Activating the artificial field produced an immediate response in the conditioned sharks. They changed from swimming steadily around the perimeter of the tank to swimming faster, turning rapidly and converging on the target in anticipation of a food reward. This change in behavior resulted in significantly more passes over the target with the artificial field on than off (ANOVA, F 2,10 =57.4,P <0.001) (Figure 2). The conditioned sharks were clearly able to sense the altered magnetic field. Candidate systems mediating this response include magnetite-based receptors, magnetically sensitive chemical reactions, or the shark electroreceptive system (or a combination of these). Unequivocal behavioral responses to earth-strength magnetic stimuli have proven elusive in most other animals but the strong response shown by the captive sharks provides a robust behavioral assay that can now be used to precisely determine how sharks detect magnetic fields and to measure detection thresholds.

Figure 2. Mean number of passes per minute by sharks across a 1.5 x 1.5 m square target during the 21 minute duration of magnetoreception experiments. Means for each 1 minute bin are derived from 11 trials. Error bars are 1 SEM.